Running multiple xserver displays “offscreen” or “headless”

x11

I would like to run two or more displays, while I only have one GPU and one monitor. The second display does not have to display anything on the monitor, a dummy output would be ideal.

I use startx to start a new display:

startx -- :1

so far so good, I can switch between the two displays on my computer using Ctrl+Alt+Fx (where x is a number, on my computer it is 7 or 8).

Here's the problem, when both displays are available and I am currently on display 0 and I try to record display 1 I get a black screen in the recorded video. The main issue seems to be that the xserver is not refreshing when the tty is not active.

I have confirmed that recording via. ffmpg's x11grab or using VLC works as intended.

Other than this I have tried Xephyr, Xnest and Xvfb, while they do that I need, I am unable to use them because what I am running in display 1 is graphically very demanding (OpenGL) and I am also recording on top of that.

I also tried using the xserver-xorg-video-dummy together with the Xdummy script to start the display, it works, but not with OpenGL applications.

Any help with this issue is greatly appreciated!

Best Answer

I found a decent workaround using Xephyr and VirtualGL!

Install both Xephyr and VirtualGL, then fire up Xephyr with your preferred settings:

Xephyr -screen 1024x768 :1

If everything went well, you will now be able to launch executables with OpenGL acceleration in Xephyr!

DISPLAY=:1 vglrun -- glxgears

Related Solutions

Screen Recording – How to Achieve Near-Perfect Quality with FFmpeg and X11

If your HDD allows, you can try to do it this way:

First write uncompressed file:

ffmpeg -f x11grab -s SZ -r 30 -i :0.0 -qscale 0 -vcodec huffyuv grab.avi

here SZ is your display size (e.g. 1920x1080).

After that you can compress it at any time you want:

ffmpeg -i grab.avi grab.mkv

Of course, you can change compression, select codec and so on.

Is it possible for a X server to have multiple displays

Quoting X(7):

From the user's perspective, every X server has a display name of the form:
hostname:displaynumber.screennumber

Each X server has one display (which may include multiple monitors, or even no monitors at all). Using multiple displays (in the X sense) requires multiple X servers; that's how you get multiple seats too.

As far as sharing goes, I think each X server expects to "own" the devices it's using at any given time, so you can't have input from a single keyboard going to multiple X servers simultaneously, or the output of multiple X servers combined on a single monitor. X servers can hand hardware off, which allows you to run X servers on multiple VTs and switch between them (this is how simultaneous logins are handled e.g. in GNOME). You can also nest some X servers (Xephyr, xpra...), so input goes to your main current X server, and gets passed on to the nested X server in a window; and the output of the nested X server is displayed in a window by the main X server.

On Linux, you could write a multiplexing input driver in the input layer to share input devices, but that's a different layer altogether than the X server.

Related Question

Display Server – Does it Start in a Display, Screen, or Monitor?